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What do you think of when you hear these words: Work… Energy… Power…
Energy • Universe is made up of matter and energy. • Energy is the mover of matter. • Energy has several forms: • Kinetic • Potential • Electrical • Heat • etc. http://tiki.oneworld.net/energy/energy.html
Energy • The Universe is made up of matter and energy. • Energy is the “mover” of matter. • There are many forms of energy. • Conversions from one form of energy to another continually occur. • Energy cannot be created or destroyed.
Work = Force|| x Distance In this case, the distance is the magnitude of the displacement. Only the component of force parallel to the displacement does work
Work • Force applied in the direction of motion • Work = Force||´ Distance • W = F||d • The unit for work is the Newton-meter which is also called a Joule. • 1 joule is equal to a force of 1 N exerted over a distance of 1 m
m= 10kg In this case, the weight does positive work d = 2 m Work = mgd = (100N)(2m) Work = 200 Nm =200J m= 10kg mg = 100N
FA m= 10kg In this case, the weight does negative work mg = 100N d = 2 m Fa Work = -mgd = -(100N)(2m) Work = -200 Nm = -200J m= 10kg mg = 100N
Power • measured in watts (W) • One watt of power is expended when one joule of work is done in one second. http://www.cartoonstock.com/newscartoons/cartoonists/pto/lowres/pton114l.jpg
Power • Power is equal to the amount of work done per unit time. • The unit for power is the Joule/second which is also called a Watt.
What is horsepower? • 1 horsepower = 746 Watts • A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute.
Mechanical Energy • Energy possessed by an object due to its motion or its stored energy of position. • It can either be potential energy or kinetic energy. • All forms of energy are measured in joules (J). • Em = Ek + Ep
Conservation of Energy • Energy cannot be created nor destroyed; it may be transformed from one form into another, but the total amount of energy never changes. • So why worry about conserving energy? http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Conservation of Mechanical Energy • Ek (before) + Ep (before) = Ek (after) + Ep (after) • When no other type of energy is present. • Why does a dropped ball not return to its original elevation? http://www.science-projects.com/Drop/DropBalls.htm
Energy Transformation of a Pendulum http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html
Potential Energy • (PE) stored chemical energy or energy of position. • An object’s ability (potential) to do work by virtue of its position. • Types: elastic, gravitational and chemical. • Examples: • Rubber bands • Springs • Bows • Batteries • Gravity?
Gravitational Potential Energy • PE = Weight ´ height • PE = m g h • Question: • How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?
Potential Energy • The same work is done on each block. What mattersis the final elevation, not thepath followed http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Kinetic Energy • The energy of motion. • Kinetic Energy = ½ mass ´ velocity2 Ek = ½ mv²
How much work does gravity do on the falling mass? PE = mgh mg h h Work = mgh mg
How much energy does the mass have at the bottom of its fall, just before it hits the ground? PE = mgh mg h h Kinetic energy mg v
PE = mgh If potential energy at top of path is 100 J, how much kinetic energy does the ball have just before it hits the ground and comes to a stop? mg h KE = ½ mv2 mg v
Check for Understanding Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s. If the roller coaster car in the last problem were moving with twice the speed, then what would be its new kinetic energy?
More Energy Transfer • High speed winds are used to do work on the blades of a turbine at the so-called wind farm. • Mech. Energy from the air gives the air particles the ability to apply a force to the blades. • As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.
Energy Transfer... • This diagram shows that the potential energy of the boy is changing as he swings. Explain what is happening, in other words how is the energy changing?
Work/Energy Relationship • If you want to move something, you have to do work. • The work done is equal (ideally) to the change in kinetic energy. • what is ideal? is this actual? • W = DKE http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Work-Energy Relationship • What happens to your stopping distance when you… • Double your speed? • Triple your speed? http://www.thegrid.co.uk/index_files/Mickel-crash-brands.jpg
Joule to Joule http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/cs.html
Forces in Car Crashes • http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html#cc3
Seatbelt Physics http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html#cc1
The formula for KE is _____? • Kinetic Energy is dependent upon ___ & ____ • If an object’s velocity doubles, its kinetic energy _____. If velocity triples, it will have _____ times the kinetic energy. • If mass doubles, kinetic energy _____. If mass triples, kinetic energy _____. • …so what happens to KE if mass doubles and velocity doubles? What if mass is cut in half and velocity doubles?
Think about standing on your desk…if you jumped off, how fast would you be moving just prior to reaching the floor? • How much work would you do on the floor? http://school.discoveryeducation.com/clipart/images/in-desk.gif
Object Falling from Rest http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2
Bungee Jumping • You wish to bungee jump off of a platform. Assuming that there is no air resistance and the spring constant of the 40 m long bungee cord is 100 N/m. How high should your platform be? • Determine all the forces acting ON the body. • Draw a free body diagram. • Apply Newton's second law. • Think about elastic potential energy. • Solve.
Momentum • Moving inertia • Symbol- p • Equation p = mv • Units: kg • m/s • VECTOR inertia In motion
A Thought Experiment: • Suppose that you were captured by an evil physicist who gave you the following choice: • You must either: • Stand in front of a 1000 kg. truck moving at 1 m/s, or • Stand in front of a 1 kg. frozen meatball moving at 1000 m/s. • …think…
Truck: • Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s • Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules • Meatball: • Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s • Meatball's kinetic energy = 0.5 mv2 = (0.5)(1 kg)(1000 m/s)2 = 500 000 Joules
Diagramming: • A 120 kg lineman moving west at 2 m/s tackles an 80 kg football fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw a vector diagram in which the before- and after-collision momenta of each player is represented by a momentum vector. Label the magnitude of each momentum vector.
Impulse • Force applied over a period of time creates impulse • Impulse equals the change in momentum
Total pbefore collision = Total pafter collision pT before = pT after (p1 + p2) before = (p1 + p2) after (m1v1 + m2v2 )before = (m1v1 + m2v2)after